Background

The most common tumors of the peripheral nerves are neuromas, which are defined as a growths or swellings on nerves. Neuromas may arise extrinsically or intrinsically. Primary intrinsic nerve cell tumors include schwannomas, which arise from Schwann cells that support the endoneurium, as well as perineuromas, which arise from cells that line the perineurium.

Fifty percent of all neuromas are found in the head and neck region. The vestibular schwannoma, also known as the acoustic neuroma, is the most frequent type of nerve cell tumor of the head and neck. Facial neuromas are extremely rare. The vast majority of facial neuromas are benign, although malignant schwannomas have occasionally been reported. Neuromas can be located intracranially, intratemporally, or extratemporally. One series of 600 temporal bones reported a facial schwannoma incidence of 0.8%.

See the image below.

A small acoustic neuroma within the internal auditory canal is easily observed on postgadolinium MRI.

Due to their low incidence, facial nerve neuromas are often misdiagnosed. Although they manifest distinctively, clinicians may still find the diagnosis difficult to establish. Patient history is usually the most reliable indicator of the presence of a facial nerve tumor. These tumors can manifest symptomatically during the early growth phase, depending on their location. Imaging studies may not reveal a tumor as a result of their small size, resulting in misdiagnosis and further loss of facial function.

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Etiology

The vast majority of intratemporal facial nerve tumors are either schwannomas or hemangiomas. Schwannomas are far more common (comprising 64% of facial nerve tumors in one retrospective series) and may arise from any point along the course of the facial nerve. These tumors tend to involve multiple facial nerve segments. Some studies report the geniculate ganglion as the most frequently involved portion of the nerve. Unlike vestibular schwannomas, no genetic locus has been implicated in the pathogenesis of facial schwannomas.
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Hemangiomas of the temporal bone are extremely rare benign vascular tumors and, when present, often arise in the area of the geniculate ganglion, although they may also be present in the internal auditory canal. The rich blood supply around the geniculate and the Scarpa ganglia may predispose these locations to hemangiomas. Unlike facial schwannomas, facial hemangiomas are extraneural in origin. They grow very slowly and are most commonly found in middle-aged adults. They tend to cause compressive symptoms even when very small in size.

Other tumors, which are less frequently observed, include perineuriomas and malignant schwannomas. Meningiomas intrinsic to the geniculate ganglion and in the intratemporal segment of the facial nerve have also been reported. Middle ear carcinoma can present with sudden deafness or facial nerve paralysis.

Hematogenous metastasis can also manifest with facial nerve involvement, disruption of the nerve sheath, and facial nerve paralysis. In descending order of frequency, the following are the most common sites for metastasis:

The intracanalicular segment

Mastoid segment

Tympanic segment

Labyrinthine segment

Tumors that have been shown to hematogenously metastasize to the temporal bone include breast, kidney, lung, and stomach tumors.

The facial nerve itself also provides a route for the direct spread of neoplastic disease into the temporal bone. Parotid mucoepidermoid carcinoma and squamous cell carcinoma of the skin have the ability to spread perineurally via the facial nerve into the temporal bone. Benign pleomorphic adenoma of the parotid gland that circumferentially involved an intratemporal segment of the facial nerve has also been reported.

Popular concern exists that cellular phone use is associated with head and neck or brain tumors. To date, case control studies have failed to show an association between regular cellular phone use and intratemporal facial nerve tumors. However, a causal relationship cannot be excluded completely based on current data. Most cellular phone exposure is brief, so dose response effects have been difficult to establish. Also, cellular phone technology is relatively new. As lifetime exposure increases, larger more powerful studies may show more convincing results.
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Pathophysiology

Most symptoms of facial nerve dysfunction due to a neoplasm are caused by compression of the nerve secondary to tumor growth. Therefore, a relatively small tumor can become symptomatic if it arises within a narrow bony canal (eg, at the labyrinthine segment), while a more proximal tumor within the cerebellopontine angle can become quite large before causing symptoms.

In addition, hemangiomas can produce facial nerve paralysis even when they are too small (several millimeters in diameter) to cause nerve compression. Some have suggested that the hemangioma shunts blood flow away from the facial nerve segment, causing local ischemia and leading to paralysis.

A study by Doshi et al of 28 patients with facial nerve schwannomas found that the tumor most frequently affected the facial nerve segment running through the internal auditory canal (19 patients, or 68%). The investigators also found that 46% of patients had multisegmental schwannomas. Hearing loss and facial weakness were the most common presentations, with the latter being most frequently linked to labyrinthine segment involvement (89%).
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Presentation

Intratemporal facial nerve tumors typically manifest with one ore more of the following symptoms: facial weakness, hearing loss, tinnitus, unsteadiness, vertigo, or pain.

Facial weakness

The incidence of facial weakness at the time of presentation varies in the literature from 51-86%. Slowly progressive (more than 3 wks) facial nerve paresis, often preceded by facial twitching, is a common presenting symptom. Note that over 50% of the facial nerve may be destroyed before clinical effects become apparent. Facial nerve paralysis is most often seen with intratemporal tumors that involve the labyrinthine segment. Extratemporal parotid tumors may also present with facial paralysis, especially if they are malignant. Recurrent episodes of facial nerve paralysis should alert the clinician to the diagnosis of a facial nerve tumor. Approximately 5% of facial nerve palsies are caused by facial nerve tumors.

For most clinicians, a history of facial paralysis related by a patient immediately triggers thoughts of Bell palsy, rather than facial nerve tumors. Because Bell palsy remains the most common diagnosis for a rapid-onset lower motor neuron facial palsy, several salient features should be considered.

Bell palsy is considered a diagnosis of exclusion in otolaryngology. Unfortunately, this exclusion can take quite a while to establish. Familiarity with the common presentation of Bell palsy is essential. Presentation is a sudden onset of unilateral lower motor nerve facial palsy or paresis that occurs over the course of 24-48 hours. It is frequently preceded or accompanied by periauricular paresthesias or even otalgia, traditionally without vesicles. The emerging conventional wisdom implicates herpes viruses, varicella-zoster, or herpes simplex. Infectious processes such as otitis media or Lyme disease must be excluded.

In some cases, a viral prodrome exists, which commonly manifests as an upper respiratory infection or as flulike symptoms. Dysgeusia, hyperacusis, and facial numbness are commonly experienced. Approximately 85% of patients recover facial function to House-Brackmann grade I or II over the course of the subsequent 8-12 weeks. In contrast, neoplastic processes most commonly cause a gradual paralysis, which exhibits no recovery. Onset of paralysis exceeds 48 hours, frequently occurring over the course of weeks. No recovery is detectable to the patient months following initial symptoms. Steroid responsiveness, if initially effective in modulating the degree of paralysis, eventually disappears and should not enter into decision-making.

Hearing loss

Hearing loss was found to be present in 46-69% of patients with intratemporal facial nerve tumors. This loss may be sensorineural (SNHL) or conductive (CHL) in nature, depending on the tumor location. Tumors proximal to the geniculate ganglion, in the labyrinthine segment, may present with SNHL, tinnitus, or vertigo. Tumors in the horizontal segment may present with conductive hearing losses.

Disclosure: Serve(d) as a director, officer, partner, employee, advisor, consultant or trustee for: Cerescan;RxRevu;Cliexa;Preacute Population Health Management;The Physicians Edge<br/>Received income in an amount equal to or greater than $250 from: The Physicians Edge, Cliexa<br/> Received stock from RxRevu; Received ownership interest from Cerescan for consulting; for: Rxblockchain;Bridge Health.